refactor(adapters): self-describing adapter pattern with auto-discovery

- Add stream_name, subject_for(), and settings_schema() to SourceAdapter ABC
- Implement all three methods in NWSAdapter, FIRMSAdapter, USGSQuakeAdapter
- Replace manual _ADAPTER_REGISTRY with pkgutil.iter_modules auto-discovery
- Remove subject_for_event from models.py (each adapter owns its subject logic)
- Update supervisor to use adapter.subject_for(event) instead of helper
- Fix quake events going to wrong stream (was publishing to CENTRAL_WX)
- Update test files to use adapter methods

This fixes the quake stream bug where events were published to
central.wx.alert.us.unknown instead of central.quake.event.<tier>.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_nws_normalization.py

@@ -17,7 +17,6 @@ from central.adapters.nws import (
     SEVERITY_MAP,
 )
 from central.config_models import AdapterConfig
-from central.models import subject_for_event
 
 
 # Sample NWS GeoJSON features for testing
@@ -272,7 +271,7 @@ class TestSubjectDerivation:
     def test_county_subject(self, adapter: NWSAdapter) -> None:
         event = adapter._normalize_feature(SAMPLE_FEATURE_ID)
         assert event is not None
-        subject = subject_for_event(event)
+        subject = adapter.subject_for(event)
         # Primary region should be alphabetically first
         # Could be county or zone depending on sort order
         assert subject.startswith("central.wx.alert.us.id.")
@@ -294,7 +293,7 @@ class TestSubjectDerivation:
         }
         event = adapter._normalize_feature(feature)
         assert event is not None
-        subject = subject_for_event(event)
+        subject = adapter.subject_for(event)
         assert "zone" in subject